Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Aug 6, 2019 at 10:36 AM Bruce Momjian <bruce@momjian.us> wrote:
> Seems reasonable (not that I am an encryption expert).
>
> > For WAL, we effectively create a 16MB bitstream, though we can create it
> > in parts as needed. (Creating it in parts is easier in CTR mode.) The
> > nonce is the segment number, but each 16-byte chunk uses a different
> > counter. Therefore, even if you are encrypting the same 8k page several
> > times in the WAL, the 8k page would be different because of the LSN (and
> > other changes), and the bitstream you encrypt/XOR it with would be
> > different because the counter would be different for that offset in the
> > WAL.
>
> But, if you encrypt the same WAL page several times, the LSN won't
> change, because a WAL page doesn't have an LSN on it, and if it did,
> it wouldn't be changing, because an LSN is just a position within the
> WAL stream, so any given byte on any given WAL page always has the
> same LSN, whatever it is.
>
> And if the counter value changed on re-encryption, I don't see how
> we'd know what counter value to use when decrypting. There's no way
> for the code that is decrypting to know how many times the page got
> rewritten as it was being filled.
>
> Please correct me if I'm being stupid here.
In my implementation (I haven't checked whether Masahiko Sawada changed this
in his patch) I avoided repeated encryption of different data using the same
key+IV by omitting the unused part of the WAL page from encryption. Already
written records can be encrypted repeatedly because they do not change.
--
Antonin Houska
Web: https://www.cybertec-postgresql.com
